The long range goal of this project is to help clarify whether malignant behavior of the cell is the result of some abnormal biochemical state or the absence of some normal characteristic(s). Cell hybridization and a mammary epithelial cell transplant system are being used to examine this critical question. Areas of interest are (1) long term culture and cloning of normal mammary epithelial cells and of malignant mammary tumor cells, (2) preparation of cell hybrids by fusion of normal cells with malignant cells, (3) transplantation of such hybrids using an in vivo system permissive (rather than selective for neoplastic cells) for growth of normal, hyperplastic or neoplastic cells, and (4) observation of the biologic characteristics of transplanted hybrid cells.